Zooms for use in the I R range are required for both civilian and military uses. The required range varies, but is generally in the 3 to 12 micron range, with some zooms being designed for only part of this range. In most of existing IR lens systems the change of magnification is effected by insertion and removal of certain lenses or lens systems or their discrete axial movement. This provides only ability for discrete change of magnification. Such designs are comparatively simple, and they operate satisfactorily at a given temperature. The systems based on the insertion and removal of lenses have to fulfill two criteria: to have an adequate resolution near the diffraction limit for all fields, while resorting to the use of a minimum of inserted and removed optical elements because of severe requirement for optical transmittance. Resulting systems are generally based on a certain compromise, providing best results only for a narrow field of view and providing a small number of lenses for wide fields but with less stringent quality requirements.
Recently, several IR zoom telescopes using two simultaneously moving groups of lenses have been developed. Thermal compensation is achieved in some of these zoom telescopes by an additional movement of at least one of the groups. In other telescopes the temperature effects are compensated for by manual focus only.
All existing IR zoom systems suffer from relative low performance for part of the magnification range. The reason for this is simple. It is impossible to change the zoom magnification while preserving the resolution and a focality when only 2 lens groups are being moved. This situation becomes worse when temperature varies. Rapid change of the refractive index of Ge causes a sharp break in the aberration balance which in turn gives rise to a deterioration of the zoom performance.
The present invention provides zoom system with high optical performance and with full automatic compensation for changes of optical properties with a change of temperatures.